1. Field of the Invention
The present invention relates to an optical fiber and a method of manufacturing the optical fiber.
2. Description of the Background Art
In accordance with the development of FTTH (Fiber To The Home), improvement in optical fiber installation efficiency has been in progress. In the installation of a FTTH system, it is often necessary to wire an optical fiber in a narrow duct or at a place available only for a small radius of curvature. Thus, optical fibers which do not cause light leakage even if they are bent have been developed, so that they can play a role of helping to enhance the efficiency of such installation. For example, there is a case in which an optical fiber is supposed to be bent at a radius of curvature of 5 mm for installation of a FTTH system, and accordingly an optical fiber which can allow such bending radius of curvature has been sought.
On the other hand, it is known that the long-term reliability of an optical fiber decreases when the optical fiber is subjected to bending at a small radius of curvature. Therefore, some techniques have been attempted such that the rigidity of a cable is strengthened to prevent optical fibers from bending substantially at a small radius of curvature or such that the elongation percentage of an optical fiber during a screening is made larger to enhance long-term reliability. However, both of such techniques not only result in increase of cost, but also become either a cause for decrease in the case of handling optical fibers as a result of enhancing the rigidity of the cable or a cause for decrease in the strength of glass constituting the optical fiber as a result of increasing the tensile stress of such screening.
A method for enhancing the strength of an optical fiber itself is disclosed in Japanese Patent Application Publication No. H 2-27308. According to the method, the outer surface of the optical fiber is coated with carbon. Since such carbon coating is done using a hydrocarbon gas, additional facilities for exhausting the gas will be needed to manufacture such optical fibers. Also, it is difficult to achieve adherence between a carbon layer and a resin layer coated over the carbon layer. If a colored layer provided for identification has a light color, the colored layer will bear a darkish color because the carbon layer is black, and consequently the identification function of the colored layer will be degraded. Moreover, in order to ensure uniformity in the carbon coating over the entire length, it is necessary to monitor the condition of the coating layer by performing an additional inspection, for example such measurement of electric parameters using the conductivity of carbon layer as mentioned in the specification of U.S. Pat. No. 5,057,781.
Also, Japanese Patent Application Publication No. H4-65327 and Japanese Patent Application Publication No. H5-124831 disclose a method for enforcing glass by providing a glass layer, such as TiO2-doped SiO2 glass or F-doped SiO2 glass whose viscosity is lower than that of SiO2, so that the glass surface of an optical fiber is transformed into a compressive stress layer. However, with such a glass whose composition is significantly different from SiO2, there is a case in which the diffusion of hydrogen easily tends to progress such that the network structure of glass is cut, resulting in degradation of breaking strength of glass in the long run because of decrease in the strength of the network structure of glass. Also, it is difficult to obtain desired optical properties since the refractive index differs from the value of SiO2 because the composition is different. Besides, it will cause a cost increase because of an additional process needed for providing a layer having another different composition over the outermost layer.
The above-mentioned methods for enhancing the strength of an optical fiber itself, either the method of applying a carbon coating around the outside surface of glass or the method of making the outside surface of glass to be a glass having a viscosity lower than SiO2, were both disadvantageous in terms of quality and manufacturing cost.